There might be a guide for this somewhere already but idk tbh

In order to unlock Steel Soul early, go into the extras tab on the main menu, and then input ⬆ ⬇ ⬆ ⬇ ⬅ ⮕ ⬅ ⮕

The game should flash and make a noise and when you attempt to create a new game it will give you both the normal and steel soul mode.

Honestly, not sure why anyone would like to purposely make this game harder on your first run, but who knows, maybe someone out there wants to play silksong like a rougelike.